The present invention relates to a sheet feeding apparatus for picking sheets one by one from a stack of sheets, and moving the picked sheets away from the stack.
Sheet feeding apparatus of this kind are commonly of either the vacuum pick or friction pick type. Vacuum pick systems use a suction member to separate the first sheet from the rest of the stack and are particularly suitable for handling sheets which are non-porous, such as currency notes in an automated teller machine (ATM). Friction type pick systems are also commonly used in ATMs. Some friction pick systems are advantageous in that they have higher feed rate capabilities than vacuum type systems and are of relatively simple construction. In addition to picking currency notes, sheet feeding apparatus of the vacuum pick or friction pick type may be used for picking other types of documents from a stack, such as photocopier sheets, tickets, vouchers, sheets of stamps, travelers cheques etc.
Friction pick systems commonly use a rotating pick roller having a high friction material disposed over its entire outer peripheral surface or over a localized area thereof. When the pick roller makes contact with a first sheet of a stack, the frictional force exerted on the sheet is greater than the frictional force between this sheet and the next sheet in the stack, which causes the first sheet to be separated from the stack and moved away by the rotating roller. However, the high friction surfaces on the picker roller tend to become worn relatively quickly and need to be replaced. This is inconvenient and expensive, as frequent maintenance of the picking mechanism is required. Moreover, the reliability of the feeding system is reduced as the friction surface becomes progressively worn.
Rotating friction belts have also been used in friction pick systems to pick sheets from a stack. One such pick apparatus is disclosed in EP-A-0559 458, where an driven endless belt is mounted on a pulley arrangement which is positioned so that, in each pick cycle, a linear portion of the rotating belt frictionally engages the first sheet of the stack, separates it therefrom, and feeds the sheet into engagement with the feed rollers of a transport mechanism which moves the sheet away from the stack. Such an arrangement is in general less susceptible to wear than friction rollers since the belt presents a larger friction area for picking. In addition, wear tends to be spread over the length of the belt rather than on a localized area thereof, since the linear portion of the belt which engages the stack is constantly changing.
Although the portion of the belt which engages the stack so as to pick a sheet therefrom, changes from one pick cycle to the next, over the lifetime of the belt, the frequency at which a particular portion of the belt engages the stack is not entirely random. During a pick operation, a belt of finite length rotates at a constant predetermined speed and a portion thereof engages the first sheet of the stack for a predetermined period of time, so as to pick the note and move it away from the stack before picking of the next note from the stack by another portion of the belt begins. A pick operation may involve a single pick cycle if only one sheet is required, or a series of successive pick cycles, if multiple sheets are required. On completion of the pick operation, the belt is brought to rest and a portion thereof remains in stationary engagement with the first sheet of the stack until a subsequent pick operation begins. Since the acceleration and deceleration times of the belt are negligible, rotation of the belt in the subsequent pick operation begins with the same portion of the belt in engagement with the stack as at the end of the previous pick operation.
Hence, over its lifetime, the portion of the belt which engages the stack during successive pick cycles begins to follow a cyclic pattern, in which some portions thereof repeatedly engage the stack while other areas of the belt never so. This results in the progressive wearing of those portions of the belt which frequently engage the stack and may eventually require replacement of the entire belt, even though substantial areas thereof are still capable of being used to perform reliable picking of sheets.